No errors in the server logs. All JavaScript loading OK.
Did notice that the WP_Piwik plugin was giving an incorrect SiteID though.
Corrected that but still couldn’t register visitors.

In the end I deleted the entire database, all the piwik files, the most recent version of WP_Piwik plugin.
Reinstalled the old 1.1.2 version of Piwik and the old version of the WP_Piwik plugin that I still had sitting on my hard drive.
Now it works! (but I can’t upgrade any of them).

Could you please try then, or in a few days, to update the WP plugin? if it does not work then, it would be apprecaited that you report the bug to the developer of the plugin so he can have a chance to fix it.